This application is a continuation-in-part of our prior application for U.S. patent, Ser. No. 25,840, filed Apr. 2, 1979, now U.S. Pat. No. 4,304,607, which is a division of our prior application for U.S. patent, Ser. No. 892,375 filed Mar. 31, 1978, and issued July 3, 1979 as U.S. Pat. No. 4,159,914, which in turn is a continuation-in-part of our prior application for U.S. patent, Ser. No. 631,815, filed Nov. 14, 1975 and issued Apr. 25, 1978 as U.S. Pat. No. 4,086,101, which in turn is a continuation-in-part of our prior application for U.S. patent, Ser. No. 508,570 filed Sept. 23, 1974, and now abandoned, which is a continuation-in-part of our prior application for U.S. patent, Ser. No. 431,705, filed Jan. 8, 1974, which issued Apr. 29, 1975 as U.S. Pat. No. 3,880,633, all of the above being assigned to a common assignee.
It is known to form photovoltaic cells by coating on a hot sheet of Nesa glass, or glass previously coated with SnO.sub.x, a thin film of CdS, by spraying a water solution of compounds which form a layer of CdS microcrystals on the SnO.sub.x, and providing a Cu.sub.x S heterojunction as a layer on the CdS layer, and forming electrodes on the film of Cu.sub.x S. In accordance with our prior applications, referred to hereinabove, the Cu.sub.x S layer may be formed by spraying a Cu.sub.x S-forming solution on the CdS film while the substrate was hot, or the Cu.sub.x S may instead be formed by dipping or by electroplating, or by a combination of both, at or near room temperature.
Photovoltaic cells have heretofore utilized relatively thick films of CdS, or have resorted to other expedients to obviate the difficulty that CdS films generally permit permeation by Cu.sub.x S and Cu, when Cu.sub.x S is formed by dipping or electroplating, i.e., by ion exchange. This permeation provides short circuits between the Cu.sub.x S layer and the SnO.sub.x, the latter constituting the negative electrode of the cell, rendering the cell inoperative. In order to convert solar energy to electrical energy on a large scale, square miles of solar cells may be required. Since Cd is a rare and expensive metal, it becomes important to form photovoltaic cells with minimum quantities of CdS and hence with extremely thin CdS films. Moreover, the cells must be reliably fabricated and have long life.
We have produced photovoltaic cells having layers of CdS and with a total thickness of about 2 to 6 microns thick, yet which show zero shorting permeation, at least one of the layers being highly resistant to permeation by Cu.sub.x S or Cu-containing solution. We have heretofore used the method of spraying a solution which forms CdS on a glass coated with SnO.sub.x, the spray being intermittent and covering only a small portion of the glass at a given point in time, while the surface of the glass is maintained at a uniform and constant temperature in the range between 230.degree. C. and 600.degree. C.
According to the present invention, in one embodiment, multiple spray applications are required to form the CdS layer. Each spray comprises a solution containing a cadmium compound and a sulfur containing compound. However, to one spray is added a metallic compound, such as AlCl.sub.3.6H.sub.2 O, in proportions such that the first layer has a metal content of at least 10 molar percent of the total metal ion content of the solution and the resulting film. In a superposed spray solution, a second layer of generally pure CdS is formed.
Instead of applying two discrete films, one formed from a spray containing a large quantity of metallic compound and the other containing little or none, a single film may be formed by gradually decreasing the metallic compound content of the solution being sprayed in proceeding from the statum of the film adjacent to the SnO.sub.x layer to the stratum of the film adjacent to the exposed surface of the CdS film. In this manner the stratum of the CdS film adjacent to the SnO.sub.x layer contains significantly greater quantities of the metal in a second compound form than the stratum of the film adjacent to the exposed surface of the CdS film.
After heat treatment at a temperature in the range of 400.degree. C. to 550.degree. C., it is found that the portion of the CdS layer containing the second metallic compound is extremely hard and highly adherent to the SnO.sub.x layer, so that it can only with difficulty be removed by application of acid or by scraping and is highly resistant to permeation by chemicals involved in forming a Cu.sub.x S layer by ion exchange, or to Cu.sub.x S and inhibits diffusion of Cu through a CdS layer. The metallic compound in the CdS film is in the relatively large quantities resulting from use of solutions containing the selected metal in a quantity representing at least 10 molar percent of the total metal ion content of the solution and does not constitute a doping procedure, such as disclosed in Middleton, et al., U.S. Pat. No. 3,411,050. Rather it comprises a compound or material having properties quite distinct from those of CdS or CdS containing only small amounts of metallic materials. It has been found that even if the entire film of CdS includes these large quantities of metallic compounds the cell remains operative, but at reduced efficiency.